Liquid development of electrostatic images

ABSTRACT

A liquid developing system for latent electrostatic images is disclosed, wherein a fine stream of liquid developer is directed upon the surface bearing the said image, relative motion being provided between the impinging stream and the surface so that a scan pattern covering the surface is defined by the linear trace of the stream striking the surface. A vacuum pickup head is positioned to follow the linear trace of applied developer, whereby excess liquid is removed from the said surface.

United States Patent Fussel [15] 3,656,173 51 Apr. 11, 1972 [54] LIQUID DEVELOPMENT OF ELECTROSTATIC IMAGES [72] Inventor: Rudi Fussel, Teaneck, NJ. [73] Assignee: Ing. C. Olivetti & C., S.p.A., lvrea, Italy [22] Filed: Aug. 8, 1969 21 App1.No.: 848,621

[52] U.S. Cl ..346/74 ES,118/637, 117/37,

2,953,470 9/1960 Green et a1. ..117/37 L X SIGNAL 6 SOURCE 3,122,453 2/1964 Montgomery ..118/637 3,371,651 3/1968 Johnson et al.... ..118/637 3,392,707 7/1968 Marx ..117/37 L X Primary ExaminerMorris Sussman Att0rneyl(evin C. McMahon [57] ABSTRACT A liquid developing system for latent electrostatic images is disclosed, wherein a fine stream of liquid developer is directed upon the surface bearing the said image, relative motion being provided between the impinging stream and the surface so that a scan pattern covering the surface is defined by the linear trace of the stream striking the surface. A vacuum pickup head is positioned to follow the linear trace of applied developer, whereby excess liquid is removed from the said surface.

4 Claims, 2 Drawing Figures IMA GE PATENTEDMR 1 I I972 IMAGE SIGNAL SOURCE BYMA f ATT INEYS LIQUID DEVELOPMENT OF ELECTROSTATIC IMAGES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to electrographic and electrophotographic apparatus and methodology, and more specifically, relates to apparatus and methodology useful in liquid development of latent electrostatic images.

2. Description of the Prior Art In the course of reproducing documents by electrostatic techniques, it becomes necessary at one point or another in the process to render visible the latent electrostatic image formed in correspondence to the original intelligence. In the well-known electrofax form of conventional electrophotography, for example, one customarily begins by uniformly charging the photoconductive surface of a zinc oxide coated paper, then selectively discharges the said surface by exposing it to a pattern of light and darkness corresponding to the intelligence to be reproduced, and thereupon develops the resultant latent electrostatic image by applying thereto a particulate colorant or tone which selectively adheres to areas of high electrostatic attraction.

Similar requirements for image development apply in those instances where the latent electrostatic image is formed electrographically i.e., by direct deposition of electrostatic charge on a dielectric. In a typical facsimile system, for example, an original document at a transmitting station is positioned on a rotatable drum. The drum is made to rotate and is axially translated with respect to an optical scanning head, which thus scans the drum in a helical pattern and transduces the optical pattern of the original to a varying electrical signal. The latter is coupled to a receiving station, at which point a sheet of dielectric-coated paper is present about a second drum rotating synchronously with that at the transmitting station. At this receiving station, a writing head takes the place of the optical scanning head, and the electrical signal from the transmitting station is transduced to a varying rate of charge deposition on the dielectric sheet, whereby to yield the desired electrostatic image. The resultant image must, of course, now be developed before it is capable of visual use, and for such purposes many of the well-known techniques for image development have in the past been used.

As is well understood by those skilled in the art, development of latent electrostatic images, as formed by the foregoing or similar techniques, will most often be accomplished by utilizing either the dry techniques of powder development or by applying to the image-bearing surface a liquid developer, by which is meant a composition including a dispersion of colored particulate matter in a dielectric liquid carrier. The latter development technique, that is to say liquid development, has found great favor in many applications, principally because of the great simplicity connected with handling and applying a liquid, and because the complexity of equipment required to achieve liquid development is rather low compared to equipment associated with effective powder development.

In those instances where liquid development is utilized, problems are normally encountered stemming from the requirement to remove excess liquid developer from the imaged surface. If removal of excess developer is not effectively accomplished, not only will excessive darkening of background areas be present, but moreover, the delivered copy will be unnecessarily wet and may have to be subjected to special drying environments, such as heating or the like, to make the copy immediately handleable. In order to remove the aforementioned excess developer, various techniques have been used in the past, including heating of the developed surface, and blotting of the surface with a roller or the like. It will be clear that a method such as heating is not only difficult to implement in a copying or facsimile machine, but moreover, may actually degrade the copy in the sense that heating does not remove toner particles, but only the liquid carrier. Similar objections apply to blotting of the image surface, and in this latter case, it is further found that smearing is likely to occur by virtue of direct contact with the blotting material. It may also be noted that in those instances where latent images are formed on dielectric paper or the like such as in the facsimile example previously cited it would be very desirable to remove excess developer from the dielectric paper while it is present on the receiving drum, thus, in effect completing the drying process in situ. Prior techniques, however, have not been readily applicable to this environment in that they are, by their nature, associated with rather bulky equipment.

OBJECTS OF THE INVENTION In accordance with the foregoing, it may be regarded as an object of the present invention to provide a liquid development system for latent electrostatic images which enables images of improved density, definition, and background characteristics.

It is a further object of the present invention to provide apparatus for liquid development of electrostatic images, which includes excess liquid removal means which are not only effective, but which will not degrade the image nor require the presence of bulky elements in the machine in which such system is incorporated.

It is a yet further object of the present invention to provide a liquid development system for electrostatic images which is particularly applicable to systems, such as facsimile or the like, wherein scanning techniques are utilized in the course of the reprographic process.

It is a still further object of the invention to provide a liquid development system for electrostatic images which is readily adaptable to developing copy sheets while the latter remain in situ at the portion of the machine where image formation occurs.

SUMMARY OF THE INVENTION Now, in accordance with the present invention, a liquid developing system for latent electrostatic images is disclosed, wherein a fine stream of liquid developer is directed upon the surface bearing the said image, relative motion being provided between the impinging stream and the surface so that a scan pattern covering the surface is defined by the linear trace of the stream striking the surface. A vacuum pickup head is positioned to follow the linear trace of applied developer, whereby excess liquid is removed from the said surface. In a preferred embodiment, the image-bearing surface is present upon a rotating drum which is axially translating with respect to the impinging developer stream, the said trace being then a helix defined by intersection of the stream with the drum surface. A vacuum nozzle is positioned beyond and point of impingement, so that excess liquid will be promptly removed as the moist trace advances to the vicinity of the nozzle orifice.

BRIEF DESCRIPTION OF DRAWINGS The invention is diagrammatically illustrated by way of example inthe appended drawings in which:

FIG. 1 is a schematic cross-section of a portion of a facsimile apparatus incorporating the present invention;

FIG. 2 is a schematic end cross-sectional view of the FIG. 1

apparatus, and illustrates the details of the development system present therein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is illustrated by way of example in FIG. 1 wherein a liquid development system in accordance with the invention is illustrated in use with a facsimile-type system. In considering FIG. 1, it should, of course, be appreciated that the facsimile system therein shown is only intended to be exemplary of a broad classof devices wherein latent electrostatic images are formed which are readily developable in accordance with the present invention.

In FIG. 1, a conductive drum 3 is shown which may be regarded as a portion of a receiving station in a facsimile system. As is well understood by those skilled in the art, a similar drum (not shown) is normally present at a sending station, the sending and receiving drums being made to rotate in synchronism as intelligence is transmitted between the sending and receiving stations. The drum 3 (as is also the case for the sending drum) is made to rotate about its axis and to translate axially with respect to a writing head 5, which in the present case, takes the form of a conductive stylus. Writing head 5 receives an electric signal from image signal source 6, which in the case of the facsimile system referred to is derived from an opticalto-electric transducer head at the sending station which is positioned thereat in a manner directly analogous to the positioning of writing head 5. The electric signal applied to writing head 5 thus varies the potential of the stylus in accordance with the optical pattern being scanned at the sending station. A sheet 7 of dielectric coated paper, such as for example the paper of this type marketed by Crown Zellerbach Corporation under the designation PS 68-531, is secured about drum 3, and as the latter rotates and translates with respect to writing head 5, a helical scan of the drum surface results and electrostatic charge is deposited on sheet 7 precisely in accord with the optical pattern present on the original document at the sending station.

In FIG. 1, it is assumed that the top of the drum is rotating in the direction of the viewer and that relative translation of the drum with respect to writing head 5 is from right to left as viewed (it being understood that such relative motion can be provided by translating head 5 while drum 3 remains axially fixed or by holding head 5 fixed and translating drum 3). Spaced slightly to the left of writing head 5, which is to say in the direction of relative drum translation, is a development nozzle 8, which is positioned (FIG. 2) with its long axis somewhat oblique with respect to the drums surface. This development nozzle is slightly spaced from the drum surface and is adapted to direct under pressure a fine stream of liquid developer upon the rotating drum bearing the imaged paper 7. The development nozzle 8 may typically comprise a simple hypodermic needle or similar device. In a representative embodiment, the nozzle 8 may be located at a distance of onehalf inch or so in the direction of paper movement from the writing head. The spacing, however, is not critical in that the latent electrostatic image being deposited on sheet 7 dissipates at a very low rate.

The liquid developer used with the invention may be any of numerous commercial products of this type as for example, that available from the Philip Hunt Chemical Co. under the designation Hunt Al-lN. Such developers as is well known in the art essentially consist of a color-supplying component, or toner, dispersed in a neutral organic liquid with high electrical resistivity and low dielectric constant.

Spaced at a very small distance further in the direction of relative paper movement is a vacuum pickup nozzle 9, which, as may be better seen in FIG. 2, is also positioned adjacent to the drum perimeter at a distance of' about one-third of the drums circumference removed from development nozzle 8. Vacuum pickup nozzle 9 is connected to a vacuum system in a manner that will be more fully described in connection with FIG. 2; the operation of the basic development system, however, will now be evident. In particular, as the drum 3 rotates and translates with respect to development nozzle 8 and vacuum pickup nozzle 9, helical traces are developed on dielectric sheet 7. A representative developer trace of this type is shown at reference numeral 10. As may be seen in FIG. 1, the spacing of pickup nozzle 9 from development nozzle 8 in the direction of relative drum translation is such that the linear trace it), as it advances from the vicinity of development nozzle 8, passes either directly or after one or more drum rotations to the vicinity of pickup nozzle 9. As the moist trace 10 passes under nozzle 9, excess toner and liquid carrier is picked up by the reduced pressure conditions present at the nozzle orifice. In particular, any excess liquid or toner which is not associated with imaged areas will be effectively removed at this point. In this connection, it should be noted that no actual physical contact is present between nozzle 9 and sheet 7, and, thus, no smearing or distortion of the image occurs. It is, of course, also evident that the process of development and removal of excess developer is enabled in situ on the drum 3, and, moreover, without subjecting the latter to harsh conditions, such as heating or the like.

As indicated above, pickup nozzle 9 can be positioned with respect to development noule 8 such that trace 10 passes directly to the vicinity of nozzle 9. That is to say that the spacing of the two nozzles in the axial direction of cylinder 3 can be small enough that the deposited trace passes to the vicinity of nozzle 8 without making a full rotation about cylinder 3. On the other hand, completely effective results are yielded where nozzle 9 follows nozzle 8 by one or more scan lines: in FIG. 1, e.g., nozzle 9 is shown some two scan lines behind nozzle 8. It may be pointed out in this connection, that when one develops by liquid spray on a rotating drum, liquid developer has a tendency to spin off the developed surface spattering adjacent ap-' paratus and wasting the liquid. By pulling the excess liquid off through the vacuum arrangement of the invention, there is not enough liquid buildup to produce a spattering problem. In-

terestingly it is possible to develop several scan lines before enough total excess liquid builds up to produce spatter problems. Thus the vacuum pickup can follow the developer nozzle by several scan lines and still yield the cited effective results.

In FIG. 2, an end cross-sectional view is shown of the FIG. 1 apparatus, and the details of the development system are illustrated. It is seen therein that liquid developer contained in the reservoir 11 is supplied under pressure to development nozzle 8 by means of an air pump 12. Adjustable valving means 13 are present in the developer delivery line 14 whereby the volume of toner per unit of time may be regulated for most effective development. It is further seen in FIG. 2 that the vacuum pickup nozzle 9 is connected via vacuum line 15 to a receiving reservoir 16. A trap 17 is present between vacuum line 15 and receiving reservoir 16, the exhaust pump 18 being directly connected to trap 17 through line 19. In operation, receiving reservoir 16 may be periodically emptied into developer reservoir 11 so that unused developer will be preserved by the system.

In a typical operation, utilizing the apparatus depicted in FIGS. 1 and 2, a base charging voltage of 7OO volts was present at writing head 5, and dielectric sheet 7 was advanced with respect to writing head 5 and nozzles 8 and 9 at some 225 ft./ min. With an axial spacing of one-half inch between head 5 and nozzle 8, and of three-sixteenths inch between nozzles 8 and 9, approximately 12 seconds elapsed between charging and development, and 3 seconds between development and excess liquid removal. The image quality yielded was excellent.

While the present invention has been particularly described in connection with its use on a rotary drum system, it will be recognized that the development system and method is equally effective with other configurations. The basic principle, that of sweeping a latent-imaged surface in a scanning mode by a liquid developer head, the linear tracing being then followed by a vacuum head which sweeps the excess developer from the moist trace, may be effectively applied, for example, to flat surfaces by scanning the latter in a spiral or rastered fashion.

Having thus disclosed the present invention, it will be evident that numerous variations upon and deviations from the invention are now enabled, and yet such var iations and deviations will continue to lie within the true scope of the present invention. Accordingly, the invention should be broadly constructed and limited only by the scope and spirit of the claims now appended hereto.

Iclaim:

1. Apparatus for writing on a rotating cylindrical surface, comprising:

a. a writing head positioned for selectively depositing electrostatic charge in image configuration on the portion of said surface passing thereunder;

b. an applicator nozzle positioned to direct a needle stream of liquid developer onto said cylindrical surface;

c. means for providing relative axial translation of said cylindrical surface for causing said stream to develop a helical trace on said cylindrical surface, said nozzle being positioned with respect to said writing head for developing a portion of said surface already imaged by said writing head;

d. a receiving nozzle fixedly spaced from said applicator nozzle in the direction of relative axial translation and adjacent said cylindrical surface, for following the helical trace of said stream upon said surface;

e. pressure reservoir means connected to said applicator nozzle for forcing said developer therethrough; and

f. vacuum means connected to said receiving nozzle for drawing excess liquid from said trace into said nozzle.

2. Apparatus for writing on a charge accepting surface comprising:

a. a writing head for selectively depositing electrostatic charge in image configuration on a portion of said surface;

b. cylindrical nozzle means for directing a fine spray of liquid developer at a point on said surface, said spray impinging on said surface at an oblique angle with a line normal to said surface at point of impact;

. means for imparting relative motion between said medium and said spray directing means for causing said spray to develop a linear trace on said surface, said nozzle means being positioned in a fixed relationship with said writing head for directing said spray at a portion of said surface already imaged by said writing head, and

d. vacuum pickup means mounted in fixed relation with said spray directing means at a point adjacent said surface and downstream along said developed trace from said spray directing means for removing excess liquid from said surface.

3. The apparatus of claim 2 wherein said means for imparting relative motion between said surface and said nozzle includes:

a. a circular cylinder, said charge accepting surface being wrapped around said cylinder;

b. means for rotating said cylinder past said nozzle means and for providing axial movement of said cylinder with relation to said spray directing means for causing said spray to develop a helical trace on said surface.

4. The apparatus of claim 2 wherein said nozzle means and said vacuum pickup means are positioned in spaced relationship with said surface. 

1. Apparatus for writing on a rotating cylindrical surface, comprising: a. a writing head positioned for selectively depositing electrostatic charge in image configuration on the portion of said surface passing thereunder; b. an applicator nozzle positioned to direct a needle stream of liquid developer onto said cylindrical surface; c. means for providing relative axial translation of said cylindrical surface for causing said stream to develop a helical trace on said cylindrical surface, said nozzle being positioned with respect to said writing head for developing a portion of said surface already imaged by said writing head; d. a receiving nozzle fixedly spaced from said applicator nozzle in the direction of relative axial translation and adjacent said cylindrical surface, for following the helical trace of said stream upon said surface; e. pressure reservoir means connected to said applicator nozzle for forcing said developer therethrough; and f. vacuum means connected to said receiving nozzle for drawing excess liquid from said trace into said nozzle.
 2. Apparatus for writing on a charge accepting surface comprising: a. a writing head for selectively depositing electrostatic charge in image configuration on a portion of said surface; b. cylindrical nozzle means for directing a fine spray of liquid developer at a point on said surface, said spray impinging on said surface at an oblique angle with a line normal to said surface at point of impact; c. means for imparting relative motion between said medium and said spray directing means for causing said spray to develop a linear trace on said surface, said nozzle means being positioned in a fixed relationship with said writing head for directing said spray at a portion of said surface already imaged by said writing head, and d. vacuum pickup means mounted in fixed relation with said spray directing means at a point adjacent said surface and downstream along said developed trace from said spray directing means for removing excess liquid from said surface.
 3. The apparatus of claim 2 wherein said means for imparting relative motion between said surface and said nozzle includes: a. a circular cylinder, said charge accepting surface being wrapped around said cylinder; b. means for rotating said cylinder past said nozzle means and for providing axial movement of said cylinder with relation to said spray directing means for causing said spray to develop a helical trace on said surface.
 4. The apparaTus of claim 2 wherein said nozzle means and said vacuum pickup means are positioned in spaced relationship with said surface. 